Process for improved coupling of rubbery polymers

ABSTRACT

Rubbery polymers made by anionic polymerization can be coupled with tin halides or silicon halides to improve the characteristics of the rubber for use in some applications, such as tire treads. In cases where the rubbery polymer was synthesized utilizing a polar modifier it is difficult to attain a high level of coupling. This invention is based upon the unexpected finding that coupling efficiency can be significantly improved by conducting the coupling reaction in the presence of a lithium salt of a saturated aliphatic alcohol, such as lithium tamylate. This invention discloses a process for coupling a living rubbery polymer that comprises reacting the living rubbery polymer with coupling agent selected from the group consisting of tin halides and silicon halides in the presence of a lithium salt of a saturated aliphatic alcohol. The lithium salt of the saturated aliphatic alcohol can be added immediately prior to the coupling reaction or it can be present throughout the polymerization and coupling process. Lithium tamylate reacts with water to form t-amyl alcohol during steam stripping. Since t-amyl alcohol forms an azeotrope with hexane, it co-distills with hexane and can contaminate recycle feed streams. This problem of recycle stream contamination can be solved by using metal salts of cyclic alcohols that do not co-distill with hexane or form compounds during steam stripping which co-distill with hexane. Thus, the use of metal salts of cyclic alcohols is preferred for this reason and because they are considered to be environmentally safe.

[0001] This is divisional of U.S. patent application Ser. No.09/461,653, filed on Dec. 14, 1999 (now pending).

BACKGROUND OF THE INVENTION

[0002] It is highly desirable for tires to exhibit good tractioncharacteristics on both dry and wet surfaces. However, it hastraditionally been very difficult to improve the tractioncharacteristics of a tire without compromising its rolling resistanceand tread wear. Low rolling resistance is important because good fueleconomy is virtually always an important consideration. Good tread wearis also an important consideration because it is generally the mostimportant factor that determines the life of the tire.

[0003] The traction, tread wear, and rolling resistance of a tire isdependent to a large extent on the dynamic viscoelastic properties ofthe elastomers utilized in making the tire tread. In order to reduce therolling resistance of a tire, rubbers having a high rebound havetraditionally been utilized in making the tire's tread. On the otherhand, in order to increase the wet skid resistance of a tire, rubberswhich undergo a large energy loss have generally been utilized in thetire's tread. In order to balance these two viscoelasticallyinconsistent properties, mixtures of various types of synthetic andnatural rubber are normally utilized in tire treads. For instancevarious mixtures of styrene-butadiene rubber and polybutadiene rubberare commonly used as a rubber material for automobile tire treads.However, such blends are not totally satisfactory for all purposes.

[0004] The inclusion of styrene-butadiene rubber (SBR) in tire treadformulations can significantly improve the traction characteristics oftires made therewith. However, styrene is a relatively expensive monomerand the inclusion of SBR is tire tread formulations leads to increasedcosts.

[0005] Carbon black is generally included in rubber compositions whichare employed in making tires and most other rubber articles. It isdesirable to attain the best possible dispersion of the carbon blackthroughout the rubber to attain optimized properties. It is also highlydesirable to improve the interaction between the carbon black and therubber. By improving the affinity of the rubber compound to the carbonblack, physical properties can be improved. Silica can also be includedin tire tread formulations to improve rolling resistance.

[0006] U.S. Pat. No. 4,843,120 discloses that tires having improvedperformance characteristics can be prepared by utilizing rubberypolymers having multiple glass transition temperatures as the treadrubber. These rubbery polymers having multiple glass transitiontemperatures exhibit a first glass transition temperature which iswithin the range of about −110° C. to −20° C. and exhibit a second glasstransition temperature which is within the range of about −50° C. to 0°C. According to U.S. Pat. No. 4,843,120, these polymers are made bypolymerizing at least one conjugated diolefin monomer in a firstreaction zone at a temperature and under conditions sufficient toproduce a first polymeric segment having a glass transition temperaturewhich is between −110° C. and −20° C. and subsequently continuing saidpolymerization in a second reaction zone at a temperature and underconditions sufficient to produce a second polymeric segment having aglass transition temperature which is between −20° C. and 20° C. Suchpolymerizations are normally catalyzed with an organolithium catalystand are normally carried out in an inert organic solvent.

[0007] U.S. Pat. No. 5,137,998 discloses a process for preparing arubbery terpolymer of styrene, isoprene, and butadiene having multipleglass transition temperatures and having an excellent combination ofproperties for use in making tire treads which comprises:terpolymerizing styrene, isoprene and 1,3-butadiene in an organicsolvent at a temperature of no more than about 40° C. in the presence of(a) at least one member selected from the group consisting oftripiperidino phosphine oxide and alkali metal alkoxides and (b) anorganolithium compound.

[0008] U.S. Pat. No. 5,047,483 discloses a pneumatic tire having anouter circumferential tread where said tread is a sulfur cured rubbercomposition comprised of, based on 100 parts by weight rubber (phr), (A)about 10 to about 90 parts by weight of a styrene, isoprene, butadieneterpolymer rubber (SIBR), and (B) about 70 to about 30 weight percent ofat least one of cis 1,4-polyisoprene rubber and cis 1,4-polybutadienerubber wherein said SIBR rubber is comprised of (1) about 10 to about 35weight percent bound styrene, (2) about 30 to about 50 weight percentbound isoprene and (3) about 30 to about 40 weight percent boundbutadiene and is characterized by having a single glass transitiontemperature (Tg) which is in the range of about −10° C. to about −40° C.and, further the said bound butadiene structure contains about 30 toabout 40 percent 1,2-vinyl units, the said bound isoprene structurecontains about 10 to about 30 percent 3,4-units, and the sum of thepercent 1,2-vinyl units of the bound butadiene and the percent 3,4-unitsof the bound isoprene is in the range of about 40 to about 70 percent.

[0009] U.S. Pat. No. 5,272,220 discloses a styrene-isoprene-butadienerubber which is particularly valuable for use in making truck tiretreads which exhibit improved rolling resistance and tread wearcharacteristics, said rubber being comprised of repeat units which arederived from about 5 weight percent to about 20 weight percent styrene,from about 7 weight percent to about 35 weight percent isoprene, andfrom about 55 weight percent to about 88 weight percent 1,3-butadiene,wherein the repeat units derived from styrene, isoprene and1,3-butadiene are in essentially random order, wherein from about 25% toabout 40% of the repeat units derived from the 1,3-butadiene are of thecis-microstructure, wherein from about 40% to about 60% of the repeatunits derived from the 1,3-butadiene are of the trans-microstructure,wherein from about 5% to about 25% of the repeat units derived from the1,3-butadiene are of the vinyl-microstructure, wherein from about 75% toabout 90% of the repeat units derived from the isoprene are of the1,4-microstructure, wherein from about 10% to about 25% of the repeatunits derived from the isoprene are of the 3,4-microstructure, whereinthe rubber has a glass transition temperature which is within the rangeof about −90° C. to about −70° C., wherein the rubber has a numberaverage molecular weight which is within the range of about 150,000 toabout 400,000, wherein the rubber has a weight average molecular weightof about 300,000 to about 800,000, and wherein the rubber has aninhomogeneity which is within the range of about 0.5 to about 1.5.

[0010] U.S. Pat. No. 5,239,009 reveals a process for preparing a rubberypolymer which comprises: (a) polymerizing a conjugated diene monomerwith a lithium initiator in the substantial absence of polar modifiersat a temperature which is within the range of about 5° C. to about 100°C. to produce a living polydiene segment having a number averagemolecular weight which is within the range of about 25,000 to about350,000; and (b) utilizing the living polydiene segment to initiate theterpolymerization of 1,3-butadiene, isoprene, and styrene, wherein theterpolymerization is conducted in the presence of at least one polarmodifier at a temperature which is within the range of about 5° C. toabout 70° C. to produce a final segment which is comprised of repeatunits which are derived from 1,3-butadiene, isoprene, and styrene,wherein the final segment has a number average molecular weight which iswithin the range of about 25,000 to about 350,000 The rubbery polymermade by this process is reported to be useful for improving the wet skidresistance and traction characteristics of tires without sacrificingtread wear or rolling resistance.

[0011] U.S. Pat. No. 5,061,765 discloses isoprene-butadiene copolymershaving high vinyl contents which can reportedly be employed in buildingtires which have improved traction, rolling resistance, and abrasionresistance. These high vinyl isoprene-butadiene rubbers are synthesizedby copolymerizing 1,3-butadiene monomer and isoprene monomer in anorganic solvent at a temperature which is within the range of about −10□C. to about 100□ C. in the presence of a catalyst system which iscomprised of (a) an organoiron compound, (b) an organoaluminum compound,(c) a chelating aromatic amine, and (d) a protonic compound; wherein themolar ratio of the chelating amine to the organoiron compound is withinthe range of about 0.1:1 to about 1:1, wherein the molar ratio of theorganoaluminum compound to the organoiron compound is within the rangeof about 5:1 to about 200:1, and herein the molar ratio of the protoniccompound to the organoaluminum compound is within the range of about0.001:1 to about 0.2:1.

[0012] U.S. Pat. No. 5,405,927 discloses an isoprene-butadiene rubberwhich is particularly valuable for use in making truck tire treads, saidrubber being comprised of repeat units which are derived from about 20weight percent to about 50 weight percent isoprene and from about 50weight percent to about 80 weight percent 1,3-butadiene, wherein therepeat units derived from isoprene and 1,3-butadiene are in essentiallyrandom order, wherein from about 3% to about 10% of the repeat units insaid rubber are 1,2-polybutadiene units, wherein from about 50% to about70% of the repeat units in said rubber are 1,4-polybutadiene units,wherein from about 1% to about 4% of the repeat units in said rubber are3,4-polyisoprene units, wherein from about 25% to about 40% of therepeat units in the polymer are 1,4-polyisoprene units, wherein therubber has a glass transition temperature which is within the range ofabout −90° C. to about −75° C., and wherein the rubber has a Mooneyviscosity which is within the range of about 55 to about 140.

[0013] U.S. Pat. No. 5,654,384 discloses a process for preparing highvinyl polybutadiene rubber which comprises polymerizing 1,3-butadienemonomer with a lithium initiator at a temperature which is within therange of about 5° C. to about 100° C. in the presence of a sodiumalkoxide and a polar modifier, wherein the molar ratio of the sodiumalkoxide to the polar modifier is within the range of about 0.1:1 toabout 10:1; and wherein the molar ratio of the sodium alkoxide to thelithium initiator is within the range of about 0.05:1 to about 10:1. Byutilizing a combination of sodium alkoxide and a conventional polarmodifier, such as an amine or an ether, the rate of polymeriztioninitiated with organolithium compounds can be greatly increased with theglass transition temperature of the polymer produced also beingsubstantially increased. The rubbers synthesized using such catalystsystems also exhibit excellent traction properties when compounded intotire tread formulations. This is attributable to the uniquemacrostructure (random branching) of the rubbers made with such catalystsystems.

[0014] U.S. Pat. No. 5,620,939, U.S. Pat. No. 5,627,237, and U.S. Pat.No. 5,677,402 also disclose the use of sodium salts of saturatedaliphatic alcohols as modifiers for lithium initiated solutionpolymerizations. Sodium t-amylate is a preferred sodium alkoxide byvirtue of its exceptional solubility in non-polar aliphatic hydrocarbonsolvents, such as hexane, which are employed as the medium for suchsolution polymerizations. However, using sodium t-amylate as thepolymerization modifier in commercial operations where recycle isrequired can lead to certain problems. These problems arise due to thefact that sodium t-amylate reacts with water to form t-amyl alcoholduring steam stripping in the polymer finishing step. Since t-amylalcohol forms an azeotrope with hexane, it co-distills with hexane andthus contaminates the feed stream.

[0015] Tire rubbers which are prepared by anionic polymerization arefrequently coupled with a suitable coupling agent, such as a tin halide,to improve desired properties. Tin-coupled polymers are known to improvetreadwear and to reduce rolling resistance when used in tire treadrubbers. Such tin-coupled rubbery polymers are typically made bycoupling the rubbery polymer with a tin coupling agent at or near theend of the polymerization used in synthesizing the rubbery polymer. Inthe coupling process, live polymer chain ends react with the tincoupling agent thereby coupling the polymer. For instance, up to fourlive chain ends can react with tin tetrahalides, such as tintetrachloride, thereby coupling the polymer chains together.

[0016] The coupling efficiency of the tin coupling agent is dependant onmany factors, such as the quantity of live chain ends available forcoupling and the quantity and type of polar modifier, if any, employedin the polymerization. For instance, tin coupling agents are generallynot as effective in the presence of polar modifiers. However, polarmodifiers such as tetramethylethylenediamine, are frequently used toincrease the glass transition temperature of the rubber for improvedproperties, such as improved traction characteristics in tire treadcompounds. Coupling reactions that are carried out in the presence ofpolar modifiers typically have a coupling efficiency of about 50-60% inbatch processes. Lower coupling efficiencies are typically attained incontinuous processes.

[0017] Each tin tetrahalide molecule or silicon tetrahalide molecule iscapable of reacting with up to four live polymer chain ends. However,since perfect stoichiometry is difficult to attain, some of the tinhalide molecules often react with less than four live polymer chainends. The classical problem is that if more than a stoichiometric amountof the tin halide coupling agent is employed, then there will be aninsufficient quantity of live polymer chain ends to totally react withthe tin halide molecules on a four-to-one basis. On the other hand, ifless than a stoichiometric amount of the tin halide coupling agent isadded, then there will be an excess of live polymer chain ends and someof the live chain ends will not be coupled. It is accordingly importantfor the stoichiometry to be exact and for all to the living polymerchain-ends to react with the coupling agent.

[0018] Conventional tin coupling results in the formation of a coupledpolymer that is essentially symmetrical. In other words, all of thepolymer arms on the coupled polymer are of essentially the same chainlength. All of the polymer arms in such conventional tin-coupledpolymers are accordingly of essentially the same molecular weight. Thisresults in such conventional tin-coupled polymers having a lowpolydispersity. For instance, conventional tin-coupled polymers normallyhaving a ratio of weight average molecular weight to number averagemolecular weight which is within the range of about 1.01 to about 1.1

[0019] U.S. Pat. No. 5,486,574 discloses dissimilar arm asymmetricradical or star block copolymers for adhesives and sealants. U.S. Pat.No. 5,096,973 discloses ABC block copolymers based on butadiene,isoprene and styrene and further discloses the possibility of branchingthese block copolymers with tetrahalides of silicon, germanium, tin orlead.

SUMMARY OF THE INVENTION

[0020] It has been unexpectedly found that coupling efficiency can besignificantly improved by conducting the coupling reactions in thepresence of a lithium salt of a saturated aliphatic alcohol, such aslithium t-amylate. In the alternative coupling efficiency can also beimproved by conducting the coupling reaction in the presence of alithium halide, or a lithium phenoxide.

[0021] This invention discloses a process for coupling a living rubberypolymer that comprises reacting the living rubbery polymer with couplingagent selected from the group consisting of tin halides and siliconhalides in the presence of a lithium salt of a saturated aliphaticalcohol. The lithium salt of the saturated aliphatic alcohol can beadded immediately prior to the coupling reaction or it can be presentthroughout the polymerization and coupling process.

[0022] Many metal salts of saturated aliphatic alcohols, react withwater to produce alcohols during steam stripping. For instance, lithiumt-amylate can react with water to produce t-amyl alcohol during steamstripping. Since t-amyl alcohol forms an azeotrope with hexane, itco-distills with hexane and can contaminate recycle feed streams. Thisproblem of recycle stream contamination can be solved by using metalsalts of cyclic alcohols that do not co-distill with hexane or formcompounds during steam stripping which co-distill with hexane. Thus, theuse of metal salts of cyclic alcohols is preferred because they solvethe problem of recycle stream contamination and are considered to beenvironmentally safe. Lithium mentholate is a highly preferred lithiumsalt of a cyclic alcohol that can be used in the practice of thisinvention.

[0023] The present invention further discloses a process for coupling aliving rubbery polymer that comprises reacting the living rubberypolymer with a coupling agent selected from the group consisting of tinhalides and silicon halides in the presence of a member selected fromthe group consisting of lithium halides and lithium phenoxides.

[0024] The subject invention also reveals a stabilized lithium initiatorsystem which is comprised of (1) an alkyl lithium compound selected fromthe group consisting of secondary alkyl lithium compounds and tertiaryalkyl lithium compounds, (2) a lithium salt of a saturated aliphaticalcohol, and (3) a hydrocarbon solvent.

DETAILED DESCRIPTION OF THE INVENTION

[0025] Virtually any type of rubbery polymer prepared by anionicpolymerization can be coupled in accordance with this invention. Infact, the techniques of this invention can be used to couple virtuallyany type of rubbery polymer synthesized by anionic polymerization. Therubbery polymers that can be coupled will typically be synthesized by asolution polymerization technique utilizing an organolithium compound asthe initiator. These rubbery polymers will accordingly normally containa “living” lithium chain end.

[0026] The polymerizations employed in synthesizing the living rubberypolymers will normally be carried out in a hydrocarbon solvent. Suchhydrocarbon solvents are comprised of one or more aromatic, paraffinicor cycloparaffinic compounds. These solvents will normally contain fromabout 4 to about 10 carbon atoms per molecule and will be liquid underthe conditions of the polymerization. Some representative examples ofsuitable organic solvents include pentane, isooctane, cyclohexane,methylcyclohexane, isohexane, n-heptane, n-octane, n-hexane, benzene,toluene, xylene, ethylbenzene, diethylbenzene, isobutylbenzene,petroleum ether, kerosene, petroleum spirits, petroleum naphtha, and thelike, alone or in admixture.

[0027] In the solution polymerization, there will normally be from 5 to30 weight percent monomers in the polymerization medium. Suchpolymerization media are, of course, comprised of the organic solventand monomers. In most cases, it will be preferred for the polymerizationmedium to contain from 10 to 25 weight percent monomers. It is generallymore preferred for the polymerization medium to contain 15 to 20 weightpercent monomers.

[0028] The rubbery polymers that are coupled in accordance with thisinvention can be made by the homopolymerization of a conjugated diolefinmonomer or by the random copolymerization of a conjugated diolefinmonomer with a vinyl aromatic monomer. It is, of course, also possibleto make living rubbery polymers that can be coupled by polymerizing amixture of conjugated diolefin monomers with one or more ethylenicallyunsaturated monomers, such as vinyl aromatic monomers. The conjugateddiolefin monomers which can be utilized in the synthesis of rubberypolymers which can be coupled in accordance with this inventiongenerally contain from 4 to 12 carbon atoms. Those containing from 4 to8 carbon atoms are generally preferred for commercial purposes. Forsimilar reasons, 1,3-butadiene and isoprene are the most commonlyutilized conjugated diolefin monomers. Some additional conjugateddiolefin monomers that can be utilized include2,3-dimethyl-1,3-butadiene, piperylene, 3-butyl-1,3-octadiene,2-phenyl-1,3-butadiene, and the like, alone or in admixture.

[0029] Some representative examples of ethylenically unsaturatedmonomers that can potentially be synthesized into rubbery polymers whichcan be coupled in accordance with this invention include alkylacrylates, such as methyl acrylate, ethyl acrylate, butyl acrylate,methyl methacrylate and the like; vinylidene monomers having one or moreterminal CH2═CH— groups; vinyl aromatics such as styrene,α-methylstyrene, bromostyrene, chlorostyrene, fluorostyrene and thelike; α-olefins such as ethylene, propylene, 1-butene and the like;vinyl halides, such as vinylbromide, chloroethane (vinylchloride),vinylfluoride, vinyliodide, 1,2-dibromoethene, 1,1-dichloroethene(vinylidene chloride), 1,2-dichloroethene and the like; vinyl esters,such as vinyl acetate; α,β-olefinically unsaturated nitriles, such asacrylonitrile and methacrylonitrile; α,β-olefinically unsaturatedamides, such as acrylamide, N-methyl acrylamide, N,N-dimethylacrylamide,methacrylamide and the like.

[0030] Rubbery polymers which are copolymers of one or more dienemonomers with one or more other ethylenically unsaturated monomers willnormally contain from about 50 weight percent to about 99 weight percentconjugated diolefin monomers and from about 1 weight percent to about 50weight percent of the other ethylenically unsaturated monomers inaddition to the conjugated diolefin monomers. For example, copolymers ofconjugated diolefin monomers with vinylaromatic monomers, such asstyrene-butadiene rubbers which contain from 50 to 95 weight percentconjugated diolefin monomers and from 5 to 50 weight percentvinylaromatic monomers, are useful in many applications.

[0031] Vinyl aromatic monomers are probably the most important group ofethylenically unsaturated monomers which are commonly incorporated intopolydienes. Such vinyl aromatic monomers are, of course, selected so asto be copolymerizable with the conjugated diolefin monomers beingutilized. Generally, any vinyl aromatic monomer which is known topolymerize with organolithium initiators can be used. Such vinylaromatic monomers typically contain from 8 to 20 carbon atoms. Usually,the vinyl aromatic monomer will contain from 8 to 14 carbon atoms. Themost widely used vinyl aromatic monomer is styrene. Some examples ofvinyl aromatic monomers that can be utilized include styrene,1-vinylnaphthalene, 2-vinylnaphthalene, α-methylstyrene,4-phenylstyrene, 3-methylstyrene and the like.

[0032] Some representative examples of rubbery polymers which can becoupled in accordance with this invention include polybutadiene,polyisoprene, styrene-butadiene rubber (SBR), α-methylstyrene-butadienerubber, α-methylstyrene-isoprene rubber, styrene-isoprene-butadienerubber (SIBR), styrene-isoprene rubber (SIR), isoprene-butadiene rubber(IBR), α-methylstyrene-isoprene-butadiene rubber andα-methylstyrene-styrene-isoprene-butadiene rubber. In cases where therubbery polymer is comprised of repeat units that are derived from twoor more monomers, the repeat units which are derived from the differentmonomers will normally be distributed in an essentially random manner.In other words, the rubbery polymer will not be a block copolymer.

[0033] The polymerizations employed in making the rubbery polymer aretypically initiated by adding an organolithium initiator to an organicpolymerization medium that contains the monomers. Such polymerizationsare typically carried out utilizing continuous polymerizationtechniques. In such continuous polymerizations, monomers and initiatorare continuously added to the organic polymerization medium with therubbery polymer synthesized being continuously withdrawn. Suchcontinuous polymerizations are typically conducted in a multiple reactorsystem.

[0034] The organolithium initiators which can be employed insynthesizing rubbery polymers which can be coupled in accordance withthis invention include the monofunctional and multifunctional typesknown for polymerizing the monomers described herein. Themultifunctional organolithium initiators can be either specificorganolithium compounds or can be multifunctional types which are notnecessarily specific compounds but rather represent reproduciblecompositions of regulable functionality.

[0035] The amount of organolithium initiator utilized will vary with themonomers being polymerized and with the molecular weight that is desiredfor the polymer being synthesized. However, as a general rule, from 0.01to 1 phm (parts per 100 parts by weight of monomer) of an organolithiuminitiator will be utilized. In most cases, from 0.01 to 0.1 phm of anorganolithium initiator will be utilized with it being preferred toutilize 0.025 to 0.07 phm of the organolithium initiator.

[0036] The choice of initiator can be governed by the degree ofbranching and the degree of elasticity desired for the polymer, thenature of the feedstock and the like. With regard to the feedstockemployed as the source of conjugated diene, for example, themultifunctional initiator types generally are preferred when a lowconcentration diene stream is at least a portion of the feedstock, sincesome components present in the unpurified low concentration diene streammay tend to react with carbon lithium bonds to deactivate initiatoractivity, thus necessitating the presence of sufficient lithiumfunctionality in the initiator so as to override such effects.

[0037] The multifunctional initiators which can be used include thoseprepared by reacting an organomonolithium compounded with amultivinylphosphine or with a multivinylsilane, such a reactionpreferably being conducted in an inert diluent such as a hydrocarbon ora mixture of a hydrocarbon and a polar organic compound. The reactionbetween the multivinylsilane or multivinylphosphine and theorganomonolithium compound can result in a precipitate which can besolubilized, if desired, by adding a solubilizing monomer such as aconjugated diene or monovinyl aromatic compound, after reaction of theprimary components. Alternatively, the reaction can be conducted in thepresence of a minor amount of the solubilizing monomer. The relativeamounts of the organomonolithium compound and the multivinylsilane orthe multivinylphosphine preferably should be in the range of about 0.33to 4 moles of organomonolithium compound per mole of vinyl groupspresent in the multivinylsilane or multivinylphosphine employed. Itshould be noted that such multifunctional initiators are commonly usedas mixtures of compounds rather than as specific individual compounds.

[0038] Exemplary organomonolithium compounds include ethyllithium,isopropyllithium, n-butyllithium, sec-butyllithium, tert-octyllithium,n-eicosyllithium, phenyllithium, 2-naphthyllithium,4-butylphenyllithium, 4-tolyllithium, 4-phenylbutyllithium,cyclohexyllithium and the like.

[0039] Exemplary multivinylsilane compounds include tetravinylsilane,methyltrivinylsilane, diethyldivinylsilane, di-n-dodecyldivinylsilane,cyclohexyltrivinylsilane, phenyltrivinylsilane, benzyltrivinylsilane,(3-ethylcyclohexyl) (3-n-butylphenyl)divinylsilane and the like.

[0040] Exemplary multivinylphosphine compounds includetrivinylphosphine, methyldivinylphosphine, dodecyldivinylphosphine,phenyldivinylphosphine, cyclooctyldivinylphosphine and the like.

[0041] Other multifunctional polymerization initiators can be preparedby utilizing an organomonolithium compound, further together with amultivinylaromatic compound and either a conjugated diene ormonovinylaromatic compound or both. These ingredients can be chargedinitially, usually in the presence of a hydrocarbon or a mixture of ahydrocarbon and a polar organic compound as a diluent. Alternatively, amultifunctional polymerization initiator can be prepared in a two-stepprocess by reacting the organomonolithium compound with a conjugateddiene or monovinyl aromatic compound additive and then adding themultivinyl aromatic compound. Any of the conjugated dienes or monovinylaromatic compounds described can be employed. The ratio of conjugateddiene or monovinyl aromatic compound additive employed preferably shouldbe in the range of about 2 to 15 moles of polymerizable compound permole of organolithium compound. The amount of multivinylaromaticcompound employed preferably should be in the range of about 0.05 to 2moles per mole of organomonolithium compound.

[0042] Exemplary multivinyl aromatic compounds include1,2-divinylbenzene, 1,3-divinylbenzene, 1,4-divinylbenzene,1,2,4-trivinylbenzene, 1,3-divinylnaphthalene, 1,8-divinylnaphthalene,1,3,5-trivinylnaphthalene, 2,4-divinylbiphenyl, 3,5,4′-trivinylbiphenyl,m-diisopropenyl benzene, p-diisopropenyl benzene,1,3-divinyl-4,5,8-tributylnaphthalene and the like. Divinyl aromatichydrocarbons containing up to 18 carbon atoms per molecule arepreferred, particularly divinylbenzene as either the ortho, meta or paraisomer and commercial divinylbenzene, which is a mixture of the threeisomers, and other compounds, such as the ethylstyrenes, also is quitesatisfactory.

[0043] Other types of multifunctional initiators can be employed such asthose prepared by contacting a sec- or tert-organomonolithium compoundwith 1,3-butadiene, at a ratio of about 2 to 4 moles of theorganomonolithium compound per mole of the 1,3-butadiene, in the absenceof added polar material in this instance, with the contacting preferablybeing conducted in an inert hydrocarbon diluent, though contactingwithout the diluent can be employed if desired.

[0044] Alternatively, specific organolithium compounds can be employedas initiators, if desired, in the preparation of polymers in accordancewith the present invention. These can be represented by R(Li)x wherein Rrepresents a hydrocarbyl radical containing from 1 to 20 carbon atoms,and wherein x is an integer of 1 to 4. Exemplary organolithium compoundsare methyllithium, isopropyllithium, n-butyllithium, sec-butyllithium,tert-octyllithium, n-decyllithium, phenyllithium, 1-naphthyllithium,4-butylphenyllithium, p-tolyllithium, 4-phenylbutyllithium,cyclohexyllithium, 4-butylcyclohexyllithium, 4-cyclohexylbutyllithium,dilithiomethane, 1,4-dilithiobutane, 1,10-dilithiodecane,1,20-dilithioeicosane, 1,4-dilithiocyclohexane, 1,4-dilithio-2-butane,1,8-dilithio-3-decene, 1,2-dilithio-1,8-diphenyloctane,1,4-dilithiobenzene, 1,4-dilithionaphthalene, 9,10-dilithioanthracene,1,2-dilithio-1,2-diphenylethane, 1,3,5-trilithiopentane,1,5,15-trilithioeicosane, 1,3,5-trilithiocyclohexane,1,3,5,8-tetralithiodecane, 1,5,10,20-tetralithioeicosane,1,2,4,6-tetralithiocyclohexane, 4,4′-dilithiobiphenyl and the like.

[0045] The polymerization temperature utilized can vary over a broadrange of from about −20° C. to about 180° C. In most cases, apolymerization temperature within the range of about 30° C. to about125° C. will be utilized. It is typically preferred for thepolymerization temperature to be within the range of about 45° C. toabout 100° C. It is typically most preferred for the polymerizationtemperature to be within the range of about 60° C. to about 85° C. Thepressure used will normally be sufficient to maintain a substantiallyliquid phase under the conditions of the polymerization reaction.

[0046] The polymerization is conducted for a length of time sufficientto permit substantially complete polymerization of monomers. In otherwords, the polymerization is normally carried out until high conversionsare attained. The polymerization is then terminated by the addition of atin halide and/or silicon halide. The tin halide and/or the siliconhalide are continuous added in cases where asymmetrical coupling isdesired. This continuous addition of tin coupling agent and/or thesilicon coupling agent is normally done in a reaction zone separate fromthe zone where the bulk of the polymerization is occurring. In otherwords, the coupling will typically be added only after a high degree ofconversion has already been attained. For instance, the coupling agentwill normally be added only after a monomer conversion of greater thanabout 90 percent has been realized. It will typically be preferred forthe monomer conversion to reach at least about 95 percent before thecoupling agent is added. As a general rule, it is most preferred for themonomer conversion to exceed about 98 percent before the coupling agentis added. The coupling agents will normally be added in a separatereaction vessel after the desired degree of conversion has beenattained. The coupling agents can be added in a hydrocarbon solution,e.g., in cyclohexane, to the polymerization admixture with suitablemixing for distribution and reaction.

[0047] The coupling agent will typically be a tin halide. The tin halidewill normally be a tin tetrahalide, such as tin tetrachloride, tintetrabromide, tin tetrafluoride or tin tetraiodide. However, tintrihalides can also optionally be used. Polymers coupled with tintrihalides having a maximum of three arms. This is, of course, incontrast to polymers coupled with tin tetrahalides which have a maximumof four arms. To induce a higher level of branching, tin tetrahalidesare normally preferred. As a general rule, tin tetrachloride is mostpreferred.

[0048] The silicon coupling agents that can be used will normally besilicon tetrahalides, such as silicon tetrachloride, silicontetrabromide, silicon tetrafluoride or silicon tetraiodide. However,silicon trihalides can also optionally be used. Polymers coupled withsilicon trihalides having a maximum of three arms. This is, of course,in contrast to polymers coupled with silicon tetrahalides which have amaximum of four arms. To induce a higher level of branching, silicontetrahalides are normally preferred. As a general rule, silicontetrachloride is most preferred of the silicon coupling agents.

[0049] A combination of a tin halide and a silicon halide can optionallybe used to couple the rubbery polymer. By using such a combination oftin and silicon coupling agents improved properties for tire rubbers,such as lower hysteresis, can be attained. In such cases, the molarratio of the tin halide to the silicon halide employed in coupling therubbery polymer will normally be within the range of 20:80 to 95:5. Themolar ratio of the tin halide to the silicon halide employed in couplingthe rubbery polymer will more typically be within the range of 40:60 to90:10. The molar ratio of the tin halide to the silicon halide employedin coupling the rubbery polymer will preferably be within the range of60:40 to 85:15. The molar ratio of the tin halide to the silicon halideemployed in coupling the rubbery polymer will most preferably be withinthe range of 65:35 to 80:20.

[0050] Broadly, and exemplary, a range of about 0.01 to 4.5milliequivalents of tin coupling agent (tin halide and silicon halide)is employed per 100 grams of the rubbery polymer. It is normallypreferred to utilize about 0.01 to about 1.5 milliequivalents of thecoupling agent per 100 grams of polymer to obtain the desired Mooneyviscosity. The larger quantities tend to result in production ofpolymers containing terminally reactive groups or insufficient coupling.One equivalent of tin coupling agent per equivalent of lithium isconsidered an optimum amount for maximum branching. For instance, if amixture tin tetrahalide and silicon tetrahalide is used as the couplingagent, one mole of the coupling agent would be utilized per four molesof live lithium ends. In cases where a mixture of tin trihalide andsilicon trihalide is used as the coupling agent, one mole of thecoupling agent will optimally be utilized for every three moles of livelithium ends. The coupling agent can be added in a hydrocarbon solution,e.g., in cyclohexane, to the polymerization admixture in the reactorwith suitable mixing for distribution and reaction.

[0051] In the practice of this invention, the coupling reaction iscarried out in the presence of a lithium compound selected from thegroup consisting of lithium salts of a saturated aliphatic alcohol, alithium halides, and lithium phenoxides. The molar ratio of the lithiumcompound to the polar modifier will typically be within the range ofabout 0.01:1 to 100:1. The molar ratio of the lithium compound to thepolar modifier will more typically be within the range of about 0.1:1 to10:1. The molar ratio of the lithium compound to the polar modifier willpreferably be within the range of about 0.4:1 to 2:1. The molar ratio ofthe lithium compound to the polar modifier will most preferably bewithin the range of about 0.7:1 to 1.4:1.

[0052] The lithium salt of the saturated aliphatic alcohol can be addedimmediately prior to coupling or it can be present during thepolymerization and coupling steps. The lithium compound can be addeddirectly as a salt of an saturated aliphatic alcohol or the salt can bemade “in-situ” by the addition of an saturated aliphatic alcohol. Forinstance, menthol can be added as a part of a lithium initiator systemand will react with organolithium compounds therein to form a lithiummentholate. It is generally preferred for the lithium salt of thesaturated aliphatic alcohol to be made by such an “in-situ” technique incommercial applications.

[0053] The lithium salt of the aliphatic alcohol can also be blendedwith the organolithium compound prior to using it as an initiator. Thisoffers a significant advantage because it stabilizes the organolithiumcompound. Additionally, it makes the lithium salt of the aliphaticalcohol much more soluble in hydrocarbon solvents. For instance,secondary alkyl lithium compounds, such as secondary-butyl lithium, andtertiary alkyl lithium compounds, such as tertiary-butyl lithium, areextremely unstable and typically must be used within 48 hours. However,it has been found that salts of saturated aliphatic alcohols can be usedto stabilize such secondary alkyl lithium compounds and tertiary alkyllithium compounds. For instance, secondary alkyl lithium compounds andtertiary alkyl lithium compounds can be stabilized with about 1 part byweight to about 100 parts by weight of a lithium salt of a saturatedaliphatic alcohol per 100 parts by weight of the secondary alkyl lithiumcompound or the tertiary alkyl lithium compound. Such compositions willtypically contain from about 10 parts by weight to about 50 parts byweight of the lithium salt of a saturated aliphatic alcohol per 100parts by weight of the secondary alkyl lithium compound or the tertiaryalkyl lithium compound. Such stabilized lithium initiator systems willtypically be dispersed in a hydrocarbon solvent.

[0054] The lithium salt of the saturated aliphatic alcohol willpreferably be a lithium alkoxide. Such lithium alkoxides are of theformula LiOR, wherein R is an alkyl group containing from about 2 toabout 12 carbon atoms. The lithium alkoxide will typically contain fromabout 2 to about 12 carbon atoms. It is generally preferred for thelithium alkoxide to contain from about 3 to about 8 carbon atoms. It isgenerally most preferred for the lithium alkoxide to contain from about4 to about 6 carbon atoms. Lithium t-amyloxide (lithium t-pentoxide) isa representative example of a preferred lithium alkoxide that can beutilized in the process of this invention.

[0055] It should be noted that even small amounts of sodium alkoxides,potassium alkoxides, cesium alkoxides, or rubidium alkoxides result inundesirable side reactions, such as chain transfer. Thus, the couplingreactions of this invention are carried out in the absence of sodiumalkoxides, cesium alkoxides, rubidium alkoxides, and potassiumalkoxides. For instance, the presence of sodium salts of saturatedaliphatic alcohols, such as sodium alkoxides, causes an undesirable jumpin Mooney viscosity and interferes with improved coupling efficiency.

[0056] As has been explained it is preferred to utilize lithium salts ofcyclic alcohols. The lithium salts of the cyclic alcohols that can bemono-cyclic, bi-cyclic or tri-cyclic. They can be substituted with 1 to5 hydrocarbon moieties and can also optionally contain hetero-atoms. Forinstance, the metal salt of the cyclic alcohol can be a metal salt of adi-alkylated cyclohexanol, such as 2-isopropyl-5-methylcyclohexanol or2-t-butyl-5-methylcyclohexanol. These salts are preferred because theyare soluble in hexane. Metal salts of disubstituted cyclohexanol arehighly preferred because they are soluble in hexane. Lithium mentholateis the most highly preferred metal salt of a cyclic alcohol that can beemployed in the practice of this invention. The metal salt of the cyclicalcohol can be prepared by reacting the cyclic alcohol directly with themetal or another metal source, such as cesium hydride, in an aliphaticor aromatic solvent.

[0057] After the coupling has been completed, a tertiary chelating alkyl1,2-ethylene diamine or a metal salt of a cyclic alcohol can optionallybe added to the polymer cement to stabilize the coupled rubbery polymer.The tertiary chelating amines that can be used are normally chelatingalkyl diamines of the structural formula:

[0058] wherein n represents an integer from 1 to about 6, wherein Arepresents an alkylene group containing from 1 to about 6 carbon atomsand wherein R′, R″, R′″ and R″″ can be the same or different andrepresent alkyl groups containing from 1 to about 6 carbon atoms. Thealkylene group A is the formula —(—CH₂—)_(m) wherein m is an integerfrom 1 to about 6. The alkylene group will typically contain from 1 to 4carbon atoms (m will be 1 to 4) and will preferably contain 2 carbonatoms. In most cases, n will be an integer from 1 to about 3 with itbeing preferred for n to be 1. It is preferred for R′, R″, R′″ and R″″to represent alkyl groups which contain from 1 to 3 carbon atoms. Inmost cases, R′, R′″, R′″ and R″″ will represent methyl groups.

[0059] A sufficient amount of the chelating amine or metal salt of thecyclic alcohol should be added to complex with any residual tin couplingagent remaining after completion of the coupling reaction.

[0060] In most cases, from about 0.01 phr (parts by weight per 100 partsby weight of dry rubber) to about 2 phr of the chelating alkyl1,2-ethylene diamine or metal salt of the cyclic alcohol will be addedto the polymer cement to stabilize the rubbery polymer. Typically, fromabout 0.05 phr to about 1 phr of the chelating alkyl 1,2-ethylenediamine or metal salt of the cyclic alcohol will be added. Moretypically, from about 0.1 phr to about 0.6 phr of the chelating alkyl1,2-ethylene diamine or the metal salt of the cyclic alcohol will beadded to the polymer cement to stabilize the rubbery polymer.

[0061] After the polymerization, coupling, and optionally thestabilization step, has been completed, the coupled rubbery polymer canbe recovered from the organic solvent. The coupled rubbery polymer canbe recovered from the organic solvent and residue by means such asdecantation, filtration, centrification and the like. It is oftendesirable to precipitate the coupled rubbery polymer from the organicsolvent by the addition of lower alcohols containing from about 1 toabout 4 carbon atoms to the polymer solution. Suitable lower alcoholsfor precipitation of the rubber from the polymer cement includemethanol, ethanol, isopropyl alcohol, normal-propyl alcohol and t-butylalcohol. The utilization of lower alcohols to precipitate theasymmetrically tin-coupled rubbery polymer from the polymer cement also“kills” any remaining living polymer by inactivating lithium end groups.After the coupled rubbery polymer is recovered from the solution,steam-stripping can be employed to reduce the level of volatile organiccompounds in the coupled rubbery polymer.

[0062] The coupled rubbery polymers that can be made by using thetechnique of this invention are comprised of a tin and/or silicon atomshaving at least three polydiene arms covalently bonded. In the case ofasymmetrically coupled rubbery polymers made by the technique of thisinvention at least one of the polydiene arms bonded to the tin atomsand/or the silicon atoms has a number average molecular weight of lessthan about 40,000, at least one of the polydiene arms bonded to the tinatoms and/or the silicon atoms has a number average molecular weight ofat least about 80,000. The ratio of the weight average molecular weightto the number average molecular weight of the asymmetrically coupledrubbery polymer will also normally be within the range of about 2 toabout 2.5.

[0063] The asymmetrically coupled rubbery polymers that can be made bythe process of this invention contain stars of the structural formula:

[0064] wherein M represents silicon or tin, wherein R₁, R₂, R₃ and R₄can be the same or different and are selected from the group consistingof alkyl groups and polydiene arms (polydiene rubber chains), with theproviso that at least three members selected from the group consistingof R₁, R₂, R₃ and R₄ are polydiene arms, with the proviso that at leastone member selected from the group consisting of R₁, R₂, R₃ and R₄ is alow molecular weight polydiene arm having a number average molecularweight of less than about 40,000, with the proviso that at least onemember selected from the group consisting of R₁, R₂, R₃ and R₄ is a highmolecular weight polydiene arm having a number average molecular weightof greater than about 80,000, and with the proviso that the ratio of theweight average molecular weight to the number average molecular weightof the asymmetrical tin-coupled rubbery polymer is within the range ofabout 2 to about 2.5. It should be noted that R₁, R₂, R₃ and R₄ can bealkyl groups because it is possible for the tin halide coupling agent toreact directly with alkyl lithium compounds which are used as thepolymerization initiator. The ratio of silicon containing stars to tincontaining stars will be within the range of about 20:80 to about 80:20in cases where the rubber is coupled with both a silicon and a tincoupling agent.

[0065] In most cases, four polydiene arms will be covalently bonded tothe tin atom or the silicon atom in the asymmetrical tin-coupled rubberypolymer. In such cases, R₁, R₂, R₃ and R₄ will all be polydiene arms.The asymmetrical tin-coupled rubbery polymer will often contain apolydiene arm of intermediate molecular weight as well as the lowmolecular weight arm and the high molecular weight arm. Suchintermediate molecular weight arms will have a molecular weight that iswithin the range of about 45,000 to about 75,000. It is normallypreferred for the low molecular polydiene arm to have a molecular weightof less than about 30,000 with it being most preferred for the lowmolecular weight arm to have a molecular weight of less than about25,000. It is normally preferred for the high molecular polydiene arm tohave a molecular weight of greater than about 90,000 with it being mostpreferred for the high molecular weight arm to have a molecular weightof greater than about 100,000. The arms of the coupled polymer willtypically be either homopolymers or random copolymers. In other words,the arms of the coupled polymers will normally not be block copolymers.

[0066] This invention is illustrated by the following examples which aremerely for the purpose of illustration and are not to be regarded aslimiting the scope of the invention or the manner in which it can bepracticed. Unless specifically indicated otherwise, all parts andpercentages are given by weight.

EXAMPLE 1

[0067] In this experiment, a tin coupled styrene-butadiene rubber wasprepared at 70° C. In the procedure used, 2300 g of asilica/alumina/molcular sieve dried premix containing 19.5 weightpercent styrene/1,3-butadiene mixture in hexanes was charged into aone-gallon (3.8 liters) reactor. The ratio of styrene to 1,3-butadienewas 15:85. After the amount of impurity in the premix was determined,2.4 ml of 1 M solution of TMEDA (N, N, N′,N′-tetramethylethylene-diamine in hexanes), 1.5 ml of 1 M solution oflithium t-butoxide (in hexanes) and 2.92 ml. of 1.03M solution ofn-butyllithium (in hexanes) were added to the reactor. The target Mn(number averaged molecular weight) was 150,000. The polymerization wasallowed to proceed at 70° C. for 1.5 hours. The GC analysis of theresidual monomers contained in the polymerization mixture indicated thatmost of the monomers were converted to polymer. After a small aliquot ofpolymer cement was removed from the reactor (for analysis), 1.2 ml. of a0.6 M solution of tin tetrachloride (in hexanes) was added to thereactor and the coupling reaction was carried out the same temperaturefor an hour. At this time, 1.0 phr (parts per 100 perts of rubber byweight) of BHT (2,6-di-tert-butyl-4-methylphenol) and 3.0 ml of 1 Msolution of TMEDA were added to the reactor to shortstop thepolymerization and to stabilize the polymer. After evaporating thehexanes, the resulting polymer was dried in a vaccum oven at 50° C. Thecoupled styrene-butadiene rubber (SBR) produced was determined to have aglass transition temperature (Tg) at −45° C. It was also determined tohave a microstructure that contained 49 percent 1,2-polybutadiene units,37 percent 1,4-polybutadiene units and 14 percent random polystyreneunits. The Mooney viscosity (ML-4) at 100° C. for this coupled polymerwas also determined to be 108.

[0068] The ML-4 for the base polymer (before coupling) was 25. Based onGPC measurement, the coupling efficiency was 80%.

EXAMPLE 2

[0069] The procedure described in Example 1 was utilized in this exampleexcept that lithium t-butoxide solution was added to the polymerizationmixture when all the monomers were consumed (90 minutes afterinitiation) and prior to adding the coupling agent. The Tg andmicrostructure of the resulting coupled SBR are shown in Table 1. TheMooney viscosities of the base and coupled polymers are also shown inTable 1. The coupling efficiency was 81%, based on GPC measurement.

Comparative Example 3

[0070] The procedure described in Example 1 was utilized in this exampleexcept that no lithium t-butoxide solution was used. The Tg andmicrostructure of the resulting coupled SBR are shown in Table 1. TheMooney viscosities of the base and coupled polymers are also shown inTable 1. The coupling efficiency was 55%, based on GPC measurement.TABLE 1 Exam- Tg ML-4 Microstructure (%) Coupling ple (° C.) BaseCoupled 1,2-PBd 1,4-PBd Styrene Efficiency 1 −45 25 108 49 37 14 80 2−44 28 115 50 36 14 81 3 −45 25 85 49 37 14 55

EXAMPLE 4

[0071] The tin coupled SBR prepared in this experiment was synthesizedin a three-reactor (1 gallon, 2 gallon, 2 gallon) continuous system at80° C. A premix containing styrene and 1,3-butadiene in hexanes wascharged into the first polymerization reactor continuously at a rate of98 grams per minute. The premix monomer solution containing a ratio ofstyrene to 1,3-butadiene of 18:82 and had a total monomer concentrationof 16%. Polymerization was initiated by adding n-butyl lithium (0.6mmole/100 grams of monomer), TMEDA (1 mmole/100 grams monomer) andlithium mentholate (0.5 mmole/100 grams monomer) to the first reactorcontinuously. The resulting polymerization medium containing the liveends was continuously pushed to the second rector (for completing thepolymerization) and then the third reactor where the coupling agent, tintetrachloride, (0.15 mmole/100 grams monomer) was added continuously.The residence time for all reactors was set at one hour to achievecomplete monomer conversion in the second reactor and complete couplingat the third reactor. The polymerization medium was continuously pushedover to a holding tank containing stabilizer and antioxidant. Theresulting polymer cement was then steam stripped and the recovered SBRwas dried in a vented oven at 50° C. The polymer was determined to havea glass transition temperature at −43° C. and have a Mooney ML-4viscosity of 82. The Mooney viscosity of base (uncoupled percurser) was41. It was also determined to have a microstructure that contained 18%random polystyrene units, 41% 1,2-polybutadiene units, and 41%1,4-polybutdiene units.

EXAMPLE 5

[0072] The procedure described in Example 4 was utilized in this exampleexcept that lithium mentholate solution was formed in-situ by reactingmenthol with n-butyllithium in a catalyst mixer loop before entering thefirst reactor. The glass transition temperature (Tg) and microstructureof the resulting coupled styrene-butadiene rubber (SBR) are shown inTable 2. The Mooney viscosities of the base and coupled polymers arealso shown in Table 2.

Comparative Example 6

[0073] The procedure described in Example 4 was utilized in this exampleexcept that no lithium alkoxide solution was used. The glass transitiontemperature and microstructure of the resulting coupledstyrene-butadiene rubber are shown in Table 2. The Mooney viscosities ofthe base and coupled polymers are also shown in Table 2. TABLE 2 Tg ML-4Microstructure (%) Example (° C.) Base Coupled 1,2-PBd 1,4-PBd Styrene 4−43 41 82 41 41 18 5 −43 38 82 40 40 18 6 −42 42 68 42 41 17

[0074] While certain representative embodiments and details have beenshown for the purpose of illustrating the subject invention, it will beapparent to those skilled in this art that various changes andmodifications can be made therein without departing from the scope ofthe subject invention.

What is claimed
 1. A stabilized lithium initiator system which iscomprised of (1) an alkyl lithium compound selected from the groupconsisting of secondary alkyl lithium compounds and tertiary alkyllithium compounds, (2) a lithium salt of a saturated aliphatic alcohol,and (3) a hydrocarbon solvent.
 2. A stabilized lithium initiator systemas specified in claim 1 wherein the alkyl lithium compound is selectedfrom the group consisting of secondary-butyl lithium and tertiary-butyllithium.
 3. A stabilized lithium initiator system as specified in claim2 wherein the lithium salt of the saturated aliphatic alcohol is alithium salt of a cyclic alcohol.
 4. A stabilized lithium initiatorsystem as specified in claim 1 wherein the lithium salt of a saturatedaliphatic alcohol is a lithium salt of a cyclic alcohol.
 5. A stabilizedlithium initiator system as specified in claim 1 wherein the lithiumsalt of the saturated aliphatic alcohol is a lithium salt of adi-alkylated cyclohexanol.
 6. A stabilized lithium initiator system asspecified in claim 1 wherein the lithium salt of the saturated aliphaticalcohol is a lithium salt of a disubstituted cyclohexanol.
 7. Astabilized lithium initiator system as specified in claim 1 wherein thelithium salt of the saturated aliphatic alcohol is lithium mentholate.8. A stabilized lithium initiator system as specified in claim 1 whereinthe alkyl lithium compound is secondary-butyl lithium.
 9. A stabilizedlithium initiator system as specified in claim 1 wherein the alkyllithium compound is tertiary-butyl lithium.
 10. A process forsynthesizing a coupled rubbery polymer which comprises (a) polymerizingat least one conjugated diolefin monomer in the presence of thestabilized lithium initiator system specified in claim 1; and (b)terminating the polymerization by the addition of a coupling agent toproduce the coupled rubbery polymer.
 11. A process as specified in claim10 wherein the coupling agent is selected from the group consisting oftin halides and silicon halides.
 12. A process as specified in claim 11wherein the coupling agent is tin tetrachloride.
 13. A process asspecified in claim 11 wherein the coupling agent is silicontetrachloride.
 14. A process as specified in claim 10 wherein thecoupling agent is a mixture of a tin halide and a silicon halide,wherein the molar ratio of the tin halide to the silicon halide iswithin the range of 20:80 to 95:5.
 15. A process as specified in claim14 wherein the molar ratio of the tin halide to the silicon halide iswithin the range of 40:60 to 90:10.
 16. A process as specified in claim15 wherein the tin halide is tin tetrachloride and wherein the siliconhalide is silicon tetra chloride.
 17. A process as specified in claim 11wherein the polymerization is terminated by the addition of a couplingagent in the presence of a polar modifier.
 18. A process as specified inclaim 17 wherein said polar modifier is selected from the groupconsisting of diethyl ether, di-n-propyl ether, diisopropyl ether,di-n-butyl ether, tetrahydrofuran, dioxane, ethylene glycol dimethylether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether,diethylene glycol diethyl ether, triethylene glycol dimethyl ether,trimethylamine, triethylamine, N,N,N′,N′-tetramethylethylenediamine,N-methyl morpholine, N-ethyl morpholine, N-phenyl morpholine, andalkyltetrahydrofurfuryl ethers.
 19. A process as specified in claim 18wherein the coupling is carried out in the absence of sodium alkoxides,cesium alkoxides, rubidium alkoxides, and potassium alkoxides.
 20. Aprocess as specified in claim 17 wherein the molar ratio of the lithiumsalt of the saturated aliphatic alcohol to the polar modifier is withinthe range of about 0.1:1 to 10:1.